Directional TOPO® cloning vectors contain a single-strand GTGG overhang on the 5´ end and a blunt end on the 3´ end. The four-nucleotide overhang invades the double-strand DNA of the PCR product and anneals to the CACC sequence that you place in your 5´ primer. Topoisomerase I then ligates the PCR product in the correct orientation.